The engine module switches the A/C compressor clutch relay through a low-side driver once the climate control requests cooling. P0645 is the general circuit fault of the relay coil control, usually an open circuit. GM Europe's 2011 Saab 9-5 sets P0645/P0646 for an open or short to ground and P0647 for a short to voltage, with the engine running and A/C requested; Ford's commercial trucks report it without the check engine light. Older Saab diesels (Bosch EDC16/PSG16) use a different assignment (P0645 short to voltage, 0.6 s). GM found a no-start with P0645 on the 2002–04 Alero/Grand Am 3.4 L because a shorted compressor clutch coil blew a fuse shared with the engine control; Ford traced warm air with P0645 on the 2020–23 Explorer to spread terminals at the body module. The compressor then does not engage or runs constantly.
Low severity. Can I keep driving?Yes, you can keep driving. The air conditioning may not work.
Symptoms
- A/C not cooling (compressor does not engage)
- Compressor runs constantly
- Check engine light usually off, code stored only
- Rarely no start if a shared fuse has blown (GM 3.4 L)
Most likely causes
- A/C clutch relay faulty (coil open)
- Control wire or connector open, terminals spread
- Relay or clutch supply fuse blown (e.g. shorted clutch coil)
- Output driver in the module faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
What each character of P0645 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 45: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0645?
Yes, you can keep driving. The air conditioning may not work.
What causes code P0645?
The most common causes are: a/C clutch relay faulty (coil open); control wire or connector open, terminals spread; relay or clutch supply fuse blown (e.g. shorted clutch coil).
Related codes
- P0646A/C Clutch Relay Control Circuit Low
- P0647A/C Clutch Relay Control Circuit High
- P0530A/C Refrigerant Pressure Sensor A Circuit
- P0532A/C Refrigerant Pressure Sensor A Circuit Low
- P0533A/C Refrigerant Pressure Sensor A Circuit High
- P0534A/C Refrigerant Charge Loss
Sources
- law.cornell.edu: 13 CCR 1968.2
- saabwisonline.com: dtc p0645 p0646 or p0647
- roushcleantech.com: Roush CleanTech Diagnostics Manual DA 8.5.18
- static.nhtsa.gov: MC 10115123 9999
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.